1. Field of the Invention
The invention in general relates to miniature switches, and more particularly, to a capacitive type MEMS switch useful in radar and other microwave applications.
2. Description of Related Art
A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar and communication systems, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance, with a low off capacitance, and a relatively low on impedance, with a high on capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint, can operate at high RF voltages and are compatible with conventional integrated circuit fabrication techniques.
Many of these MEMS switches generally have electrostatic elements, such as opposed electrodes, which are attracted to one another upon application of a DC pull down control voltage. An opposed electrode is defined on the underside of a two-arm moveable bridge above the pull down electrode. Upon application of the DC pull down control voltage, the bridge is deflected down and, by the particular high capacitive coupling established, the electrical impedance is significantly reduced between first and second spaced apart RF conductors on a substrate member, thus allowing a signal to propagate between the first and second conductors.
In the capacitive type MEMS switch, a dielectric layer is deposited on the first conductor in an area opposite the underside of the two-arm moveable bridge, with this area on the conductor acting as the pull down electrode. With this arrangement, the full pull down voltage appears across the dielectric layer resulting in a relatively high electric field across the dielectric. This high field leads to charge accumulation on the surface, as well as in the bulk dielectric. Once the dielectric accumulates enough charge, the switch will fail because the charge causes the switch to remain closed even after the pull down voltage is removed.
Another problem with conventional capacitive type MEMS switches occurs while the switch closes. The high electric field across the dielectric increases exponentially as the bridge moves toward the dielectric until it makes contact with the dielectric. The high field generated during the switch closing causes metal to deposit onto the dielectric from the bridge, thus degrading the value of capacitance in the closed position to unacceptable values.
In addition, work has been done in the development of metal-to-metal MEMS switches where the moving metal bridge directly contacts the bottom metal plate. This type of switch is useful because it works over wide bandwidths, however the metal contacts wear after around one million cycles. The advantage of the capacitive type switch is that the presence of the dielectric layer avoids the wearing out of the switch contacts and has shown operation of around ten billion cycles.
The present invention obviates these objectionable charging and metal deposition problems in a capacitive type MEMS switch.
A capacitive type MEMS switch is described and comprises a substrate member with a conductor arrangement deposited on the substrate. The conductor arrangement includes first and second RF conductors and having a dielectric layer deposited on a portion of the conductor arrangement. A bridge member is positioned over a portion of the conductor arrangement and has a central portion with first and second arms extending out from the central portion and supported by respective first and second support members. The conductor arrangement defines an open area, and a pull down electrode, of a height less that that of the conductor arrangement, is positioned within this open area, and is substantially surrounded by the conductor arrangement. During operation the central portion of the bridge member is drawn toward the conductor arrangement upon application of a control voltage to the pull down electrode, to present a relatively low impedance, allowing a signal to propagate between the first and second RF conductors. The invention described here removes any DC voltage from the dielectric used in the switch. The pull down voltage is between the top metal and the pull down electrode with air inbetween. This eliminates the dielectric charging which plagues capacitance type MEMs switches, and also eliminates the deposition of material onto the dielectric surface which degrades the down capacitance.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description.